1. Field of the Invention
This invention relates to a centrifugal liquid barrier with pressure responsive loading means. More particularly, the invention relates to pressure balanced face seals suitable for vertical pumps applied in a high pressure and high temperature environment.
2. Summary of the Prior Art
A dynamic pump seal basically comprises a stationary member coacting with a rotating member to control or prevent the leakage of fluids along a rotating shaft. A variety of dynamic seals are known including, illustratively, face contact, controlled leakage, labyrinth, visco and centrifugal seals. The features which characterize the different seals may be used separately or in combination. In a static seal, contrastingly, no relative motion exists between the seal and the mating surface to be sealed.
Dynamic seals are popularly further categorized as face seals where the members are disposed with the sealing interface orientated transversely to the longitudinal axis of the shaft. The stationary and rotating members are separated by a thin hydrodynamic fluid film between the members, thereby insuring lower friction and increasing seal life. The thin film of liquid between the seal faces not only lubricates these surfaces but provides a force keeping the surfaces from actual contact. It is customary to have one of the sealing members rigidly mounted and the other mounted to float, i.e., to permit axial and angular motion. Axial forces, mechanically and hydraulically applied, maintain the faces in close proximity such that the film thickness value between the seal faces is usually less than five-thousands of an inch with values of 20 to 100 microinches being common. Since a gap and pressure differential exists, there will be some leakage. This leakage is incipient, however, because of the extremely small clearance. Since film thickness values are so small, the flatness of the seal faces is extremely important in minimizing wear. For this reason, the seal faces are usually precision lapped until they are flat to within one helium light band, i.e., about 12 microinches, to eliminate asperities. Face seal leakage is a function of the axial forces acting on the sealing members. In a face seal, these forces may press an axially floating non-rotating ring against a fixed running counterface or an axially floating rotating ring against a fixed stationary counterface.
The axial leakage path between the floating member and the shaft is generally closed by a secondary static seal, such as an O-ring, which allows primary movement of the primary seal without axial leakage. Face seals are satisfactory so long as friction and wear are not excessive. As speed and pressure increase, however, rubbing contacts become less tolerable and the maintenance of a fluid film between the seal faces more imperative. To limit the unit loading on the coacting faces, most high-pressure seals are hydraulically balanced. Balancing is the geometric arrangement of the seal assembly to lower the load between the rotating and stationary faces. By varying the diameters of each seal member, specified unit pressures can be attained at the seal interface. Typically, the closing force on the floating member barely overcomes the opening force created by the pressure between the seal faces to provide a minimum unit loading between coacting seal faces, thereby, promoting seal life. In high pressure applications, in order to further reduce the acting on the seal and to prevent leakage, multiple face contact seals have been spaced axially along the shaft and the seals which are adjacent to each other bridged by pressure reducing means to provide a reduced pressure differential across the seal faces.
In many applications, the trend has been to use the system thermodynamic working fluid which is being sealed as the lubricating film to overcome possible contamination problems and, in nuclear applications in particular, the breakdown of standard lubricants as a result of radiation.
Face seals have been utilized in nuclear power systems for sealing large vertical reactor coolant pumps. Vertical pumps have a shaft axis which is vertically disposed. Reactor coolant pumps generally circulate water through the reactor, steam generator, and associated piping, developing the requisite head to overcome fluid friction losses and to transport heat from the nuclear reactor to the steam generators. In pressurized water reactor systems, a pressurizer may be utilized to establish and maintain a system pressure in excess of 2000 psia. The high pressure is required to prevent boiling at high system temperatures typically in excess of 500.degree. F. The reactor coolant pump seals, therefore, must be designed to operate under these conditions.
The availability of nuclear reactors has been limited due to the frequent necessity to perform repairs on reactor coolant pump seals. Operational problems encountered with seals in reactor coolant pumps have included excessive leakage, heat checking of the rotating member, excessive secondary seal wear, shaft sleeve fretting or wear, uneven wear of the stationary seal member, sensitivity to temperature changes, secondary seal hang-up, frequent destaging and sensitivity to pump shaft motion.
Many of the presently operating reactor coolant pumps are designed such that the seals are not located near a bearing. This design arrangement appears to be uniquely predominant only in the large vertical type pumps being used in nuclear reactor systems. Due to the three bearing arrangement presently found in most reactor coolant pump-motor combinations, pump shaft lateral displacements are much greater than are traditionally found on machinery. In addition, the seals are positioned near the point of greatest shaft displacement. Also, it is known that the pump shaft moves up or down due to changes in the pump's axial thrust. When a running reactor coolant pump is secured, for example, the normal impeller down thrust is discontinued, and an increased net upward force is exerted due to high reactor coolant system pressure. At low system pressure, the weight of the shaft causes a downward thrust. Total axial shaft motion has been measured and found to vary from 80 mils to 120 mils depending upon the type of motor employed and conditions in the reactor coolant system. Only approximately 10 mils is due to motor thrust bearing clearances, the rest being attributable to motor housing and bearing support deflections and thermal growth.
Dynamic lateral shaft displacements have been found to vary in operating plants over a range of 5 to 22 mils during steady state operations. Depending upon the manufacturing and alignment tolerances, shaft displacement will vary from pump to pump. Moreover, there will always be a tendency for some shaft displacement due to the radial load at the impeller. Shaft vibration and runout also contribute to shaft displacement although these phenomena may counteract the generally stationary force attributed to radial thrust.
Hence, it appears that although seal technology is well advanced, the effects of the operating environment and bearing arrangement in nuclear reactor coolant pump applications have combined to yield less than satisfactory seal performance. Optimal seal development has, heretofore, been hindered by failure to synthesize recently recognized phenomena affecting seal performance with those which are more readily apparent.
The recently recognized phenomena, which are characterized herein as hydraulic moment unbalance and secondary seal loading and cyclic motion, and which are fully described hereinafter, result in rapid wear of the seal faces and instability of the axially movable member as well as rapid degradation of the secondary seal. The phenomena are associated with the bearing arrangement described hereinbefore.
Other readily recognizable phenomena have acted to confound efforts to achieve satisfactory seal performance in conjunction with hydraulic moment unbalance and secondary seal cyclic motion. These known phenomena include seal ring moment deflections due to thermal, hydraulic or mechanical loads, seal wobble due to shaft tilt or seal housing deflections, and the inability of the floating member to dynamically track its mating seal member during shaft axial displacement.
Accordingly, there is a need for a seal, suitable for use in reactor coolant pumps, which is not affected by hydraulic moment unbalance and otherwise satisfactorily performs its sealing function.